How to reproduce:
1. Open the attached '1305938.zip' project
2. Open 'SampleScene'
3. Open Tools > ProBuilder > ProBuilder Window
4. With the Object Selection tool select the Cube object
5. Observe the ProBuilder window

Expected result: The ProBuilderize option is not greyed out
Actual result: The ProBuilderize option is greyed out

Reproducible with: ProBuilder 5.0.0-pre.7 (2019.4.18f1, 2020.2.2f1, 2021.1.0b3, 2021.2.0a2)
Not reproducible with: ProBuilder 4.4.0 ( 2018.4.31f1, 2019.4.18f1, 2020.2.2f1, 2021.1.0b3, 2021.2.0a2)
Cannot test with: ProBuilder 5.0.0-pre.7 (2018.4.31f1) (the package version is not supported)

Notes:
- The issue is not reproducible with the Vertex Selection, Edge Selection, and Face Selection tools